fix: PoSt worker: use go-state-types for proof policies

[arajasek]

Related Issues

@siriusyim noticed this bug, and opened #10715 to fix it

Proposed Changes

Backport it onto the 1.22.0 release branch.

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
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• Tests exist for new functionality or change in behavior
• CI is green

[arajasek]

(Approved by @magik6k on #10716 )